Wind and rain belts to shift north as planet warms, research says

September 23, 2013

During boreal summer, Earth's tropical rain belt migrates north. A similar but prolonged shift could happen if the north continues to heat faster than the south, disrupting global rainfall patterns. Credit: Mats Halldin

As humans continue to heat the planet, a northward shift of Earth's wind and rain belts could make a broad swath of regions drier, including the Middle East, American West and Amazonia, while making Monsoon Asia and equatorial Africa wetter, says a new study in Proceedings of the National Academy of Sciences.

The study authors base their prediction on the warming that brought Earth out of the last ice age, some 15,000 years ago. As the North Atlantic Ocean began to churn more vigorously, it melted Arctic sea ice, setting up a temperature contrast with the southern hemisphere where sea ice was expanding around Antarctica. The temperature gradient between the poles appears to have pushed the tropical rain belt and mid-latitude jet stream north, redistributing water in two bands around the planet.

Today, with Arctic sea ice again in retreat, and the northern hemisphere heating up faster than the south, history could repeat itself. "If the kinds of changes we saw during the deglaciation were to occur today that would have a very big impact," said the study's lead author, Wallace Broecker, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory.

Marshaling climate data collected from around the world, from tree-rings, polar ice cores, cave formations, and lake and ocean sediments, Broecker and study coauthor, Aaron Putnam, a climate scientist at Lamont-Doherty, hypothesize that the wind and rain belts shifted north from about 14,600 years ago to 12,700 years ago as the northern hemisphere was heating up.

At the southern edge of the tropical rain belt, the great ancient Lake Tauca in the Bolivian Andes nearly dried up at this time while rivers in eastern Brazil slowed to a trickle and rain-fed stalagmites in the same region stopped growing. In the middle latitudes, the northward advance of the jet stream may have caused Lake Lisan, a precursor to the Dead Sea in Jordan's Rift Valley, to shrink, along with several prehistoric lakes in the western U.S., including Lake Bonneville in present day Utah.

Meanwhile, a northward shift of the tropical rains recharged the rivers that drain Venezuela's Cariaco Basin and East Africa's Lake Victoria and Lake Tanganyika. Stalagmites in China's Hulu Cave grew bigger. Evidence for a stronger Asian monsoon during this time also shows up in the Greenland ice cores.

The process worked in reverse from about 1300 to 1850, the study authors hypothesize, as northern Europe transitioned from the relatively warm medieval era to a colder period known as the Little Ice Age. Ocean circulation slowed, and sea ice in the North Atlantic Ocean expanded, the climate record shows. At the same time, rainfall declined in Monsoon Asia, leading to a series of droughts that have been linked to the decline of Cambodia's ancient Khmer civilization, China's Ming dynasty and the collapse of kingdoms in present day Vietnam, Myanmar and Thailand.

Western China is growing drier, turning lakes like this one to dust. If wind and rain patterns shift north as the study predicts, drying in this region could continue. Credit: David Putnam

In the southern hemisphere, the reconstruction of glacier extents in New Zealand's Southern Alps suggests that the mid-latitudes may have been colder during medieval times, supporting the idea of a temperature contrast between the hemispheres that altered rain and wind patterns.

A similar migration of Earth's wind and rain belts happens each year. During boreal summer, the tropical rain belt and mid-latitude jet stream migrate north as the northern hemisphere heats up disproportionately to the south, with more continents to absorb the sun's energy. As the northern hemisphere cools off in winter, the winds and rains revert south.

Sometimes the winds and rains have rearranged themselves for longer periods of time. In the 1970s and 1980s, a southward shift of the tropical rain belt, attributed to air pollution cooling the northern hemisphere, is thought to have brought devastating drought to Africa's Sahel region. The tropical rain belt has since reverted back, and may be moving north, the study authors say, as suggested by a number of recent droughts, including in Syria, northern China, western U.S., and northeastern Brazil.

Consistent with the study, at least one climate model shows the tropical rain belt moving north as carbon dioxide levels climb and temperatures warm. "It's really important to look at the paleo record," said Dargan Frierson, an atmospheric scientist at University of Washington whose modeling work supports the authors' hypothesis. "Those changes were huge, just like we're expecting with global warming."

The study authors acknowledge that their hypothesis has some holes. In the past, changes in sea ice cover drove the temperature gradient between the two hemispheres while today rapidly rising industrial carbon emissions are responsible. So far, there is also no clear evidence that ocean circulation is increasing in the North Atlantic or that the monsoon rains over Asia are strengthening (though there is speculation that sulfate aerosols produced by burning fossil fuels may be masking this effect).

As air pollution in the northern hemisphere declines, temperatures may warm, creating the kind of temperature contrast that could move the winds and rains north again, said Jeff Severinghaus, a climate scientist at Scripps Institution of Oceanography who was not involved in the study.

"Sulfate aerosols will probably get cleaned up in the next few decades because of their effects on acid rain and health," he said. "So Broecker and Putnam are probably on solid ground in predicting that northern warming will eventually greatly exceed southern warming."

Related Stories

Scientists still puzzle over how Earth emerged from its last ice age, an event that ushered in a warmer climate and the birth of human civilization. In the geological blink of an eye, ice sheets in the northern hemisphere ...

In a band near the equator, southward Northern Hemisphere winds and northward Southern Hemisphere winds come together to form the Intertropical Convergence Zone (ITCZ). In the ITCZ the equatorward winds rise and cool, creating ...

In the northern hemisphere, ice sheets ebb and flow in 100,000-year cycles, driven by varying amounts of sunlight falling on Earth's surface as its orbit and orientation toward the sun changes. But astronomical variations ...

(Phys.org) —Much attention is paid to melting sea ice in the Arctic. But less clear is the situation on the other side of the planet. Despite warmer air and oceans, there's more sea ice in Antarctica now than in the 1970s ...

Recommended for you

At the end of the Pleistocene period, approximately 12,800 years ago—give or take a few centuries—a cosmic impact triggered an abrupt cooling episode that earth scientists refer to as the Younger Dryas.

In a new assessment of nine state-of-the-art climate model simulations provided by major international modeling centers, Michael Rawlins at the University of Massachusetts Amherst and colleagues found broad disagreement in ...

New research confirms that the land under the Chesapeake Bay is sinking rapidly and projects that Washington, D.C., could drop by six or more inches in the next century—adding to the problems of sea-level rise.

The world's deserts may be storing some of the climate-changing carbon dioxide emitted by human activities, a new study suggests. Massive aquifers underneath deserts could hold more carbon than all the plants on land, according ...

Wildfires in California's fabled Sierra Nevada mountain range are increasingly burning high-elevation forests, which historically have seldom burned, reports a team of researchers led by the John Muir Institute of the Environment ...

But ubavontuba, we all know that the planet continues to warm because all the heat is going into the deep ocean. We don't need evidence for this or even a way of measuring it because unicorns told us and they're always right!

Yea, verily, as prophesies are, no higher powers are required to make it, and a mirror to the South expecting the abundance of the Holy Element there, and nore Earth North.That it is observed is a result.

@ubavontuba et al, 10 of the warmest years on record have occurred in the last 15 years. Please explain how this is consistent with your assertion that warming has 'stopped'. http://en.wikiped...st_years

@ubavontuba et al, 10 of the warmest years on record have occurred in the last 15 years. Please explain how this is consistent with your assertion that warming has 'stopped'. http://en.wikiped...st_years

Stop mentioning facts! Facts are for leftists, because reality has a well-documented liberal bias! The climate negationists aren't worth bothering with, they are completely insensitive to reason.

I found this article puzzling and hard to follow. I am told that during boreal summer (I assume that's the northern hemisphere's summer, in layman's terms?), the Earth's tropical rain belt migrates north. The graphic shows a red belt and a blue belt, but has no explanation of what they represent. I'm gonna guess that they show the location of a single rain belt - red showing the position during northern summer and blue showing the position of the same belt during the southern summer. If you ask me, that could just as easily mean a southern migration of the belt when its summer in the southern hemisphere. So again, I must assume what the authors really mean is a there's a general northward movement of the average position of the tropical rainbelt? The whole article is full of examples of drying at various locations around the globe, without any reference to where those locations sit in relation to the 'rainbelt'. All I can say is - there's a whole lotta lot postulation goin' on!

@ubavontuba et al, 10 of the warmest years on record have occurred in the last 15 years. Please explain how this is consistent with your assertion that warming has 'stopped'. http://en.wikiped...st_years

Isn't it obvious? Think about it like this: Which direction is your car going, relative to the surface of the earth, when it is stopped at a stop sign?

Weather Channel showed how the tornado belt is shifting south and east.Tornadoes are spawned by the jet stream"Tornado Alley" is defined by the American Meteorological Society as: "A term often used by the media to denote a zone in the Great Plains region of the central United States, often a north-south oriented region centered on north Texas, Oklahoma, Kansas, and Nebraska, where tornadoes are most frequent. Since statistics are variable on all timescales, the term has little scientific value."(1) However, over the past decade, a noticeable increase in tornado activity has been seen outside of this area, spreading farther east into the Mid-Mississippi Valley and Mid-South. This increase in tornado activity seems to center around and south of our station in Paducah, KY, which covers parts of southern IL, southeast MO, west KY, west TN, and a small part of southwest IN. "http://www.thewea...dex.html

The facts remain, "the polar bears will be fine". Freeman Dyson (who is not a crank, or cracked, nor does he have a political axe to grind. Additionally, he is smarter, and knows more about climatology, than any of you (by several orders of magnitude)).

The facts remain, "the polar bears will be fine". Freeman Dyson (who is not a crank, or cracked, nor does he have a political axe to grind. Additionally, he is smarter, and knows more about climatology, than any of you (by several orders of magnitude)).

Apparently, the recycled talking points, appeals-to-(dubious)authority and evidence-free put downs are anything but extinct, at least in your play book.

Please sign in to add a comment.
Registration is free, and takes less than a minute.
Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.